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In
organic chemistry Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, J ...
, a thiol (; ), or thiol derivative, is any organosulfur compound of the form , where R represents an
alkyl In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term ''alkyl'' is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of . A cycloalkyl is derived from a cycloa ...
or other organic
substituent A substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. (In organic chemistry and biochemistry, the terms ''substituent'' and ''functional group'', as well as '' side ...
. The
functional group In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the r ...
itself is referred to as either a thiol group or a sulfhydryl group, or a sulfanyl group. Thiols are the sulfur analogue of alcohols (that is, sulfur takes the place of
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...
in the
hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydrox ...
() group of an alcohol), and the word is a blend of "''thio-''" with "alcohol". Many thiols have strong odors resembling that of
garlic Garlic (''Allium sativum'') is a species of bulbous flowering plant in the genus '' Allium''. Its close relatives include the onion, shallot, leek, chive, Welsh onion and Chinese onion. It is native to South Asia, Central Asia and northeas ...
or rotten eggs. Thiols are used as
odorants An aroma compound, also known as an odorant, aroma, fragrance or flavoring, is a chemical compound that has a smell or odor. For an individual chemical or class of chemical compounds to impart a smell or fragrance, it must be sufficiently v ...
to assist in the detection of
natural gas Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbon d ...
(which in pure form is odorless), and the "smell of natural gas" is due to the smell of the thiol used as the odorant. Thiols are sometimes referred to as mercaptans () or mercapto compounds, a term introduced in 1832 by William Christopher Zeise and is derived from the
Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...
('capturing mercury')''Oxford American Dictionaries'' ( Mac OS X Leopard). because the thiolate group () bonds very strongly with mercury compounds.


Structure and bonding

Thiols of the structure R−SH are referred to as Alkanethiols or Alkyl thiols, in which an
alkyl In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term ''alkyl'' is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of . A cycloalkyl is derived from a cycloa ...
group (R) is attached to a sulfhydryl group (SH). Thiols and alcohols have similar connectivity. Because sulfur atoms are larger than oxygen atoms, the C−S bond lengths – typically around 180 picometres in length – are about 40 picometers longer than a typical C−O bond. The C−S−H angles approach 90° whereas the angle for the C−O−H group is more obtuse. In the solid or liquids, the hydrogen-bonding between individual thiol groups is weak, the main cohesive force being Van der Waals interactions between the highly polarizable divalent sulfur centers. The S−H bond is much weaker than the O−H bond as reflected in their respective bond dissociation energy (BDE). For CH3S−H, the BDE is , while for CH3O−H, the BDE is . Due to the small difference in the electronegativity of sulfur and hydrogen, an S−H bond is moderately
polar Polar may refer to: Geography Polar may refer to: * Geographical pole, either of two fixed points on the surface of a rotating body or planet, at 90 degrees from the equator, based on the axis around which a body rotates *Polar climate, the cli ...
. In contrast, O−H bonds in hydroxyl groups are more polar. Thiols have a lower dipole moment relative to their corresponding alcohols.


Nomenclature

There are several ways to name the alkylthiols: * The suffix ''-thiol'' is added to the name of the alkane. This method is nearly identical to naming an alcohol and is used by the
IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
, e.g. CH3SH would be '' methanethiol''. * The word ''mercaptan'' replaces ''alcohol'' in the name of the equivalent alcohol compound. Example: CH3SH would be methyl mercaptan, just as CH3OH is called methyl alcohol. * The term ''sulfhydryl-'' or ''mercapto-'' is used as a prefix, e.g. mercaptopurine.


Physical properties


Odor

Many thiols have strong odors resembling that of
garlic Garlic (''Allium sativum'') is a species of bulbous flowering plant in the genus '' Allium''. Its close relatives include the onion, shallot, leek, chive, Welsh onion and Chinese onion. It is native to South Asia, Central Asia and northeas ...
. The odors of thiols, particularly those of low molecular weight, are often strong and repulsive. The spray of skunks consists mainly of low-molecular-weight thiols and derivatives. These compounds are detectable by the human nose at concentrations of only 10 parts per billion. Human sweat contains (''R'')/(''S'')-3-methyl-3-mercapto-1-ol (MSH), detectable at 2 parts per billion and having a fruity,
onion An onion (''Allium cepa'' L., from Latin ''cepa'' meaning "onion"), also known as the bulb onion or common onion, is a vegetable that is the most widely cultivated species of the genus '' Allium''. The shallot is a botanical variety of the on ...
-like odor. (Methylthio)methanethiol (MeSCH2SH; MTMT) is a strong-smelling volatile thiol, also detectable at parts per billion levels, found in male mouse urine.
Lawrence C. Katz Lawrence C. Katz (December 23, 1956 – November 26, 2005) was an American neurobiologist. He was an investigator at the Howard Hughes Medical Institute. His lab was located in Duke University Medical Center, where he was the James B. Duke Prof ...
and co-workers showed that MTMT functioned as a semiochemical, activating certain mouse olfactory sensory neurons, attracting female mice.
Copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pink ...
has been shown to be required by a specific mouse olfactory receptor, MOR244-3, which is highly responsive to MTMT as well as to various other thiols and related compounds. A human olfactory receptor, OR2T11, has been identified which, in the presence of copper, is highly responsive to the gas odorants (see below) ethanethiol and ''t''-butyl mercaptan as well as other low molecular weight thiols, including allyl mercaptan found in human
garlic Garlic (''Allium sativum'') is a species of bulbous flowering plant in the genus '' Allium''. Its close relatives include the onion, shallot, leek, chive, Welsh onion and Chinese onion. It is native to South Asia, Central Asia and northeas ...
breath, and the strong-smelling cyclic sulfide thietane. Thiols are also responsible for a class of wine faults caused by an unintended reaction between sulfur and
yeast Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constit ...
and the "skunky" odor of beer that has been exposed to ultraviolet light. Not all thiols have unpleasant odors. For example,
furan-2-ylmethanethiol Furan-2-ylmethanethiol (2-Furanmethanethiol) is an organic compound containing a furan substituted with a sulfanylmethyl group. It is a clear colourless liquid when pure, but it becomes yellow coloured upon prolonged standing. It possesses a stro ...
contributes to the aroma of roasted
coffee Coffee is a drink prepared from roasted coffee beans. Darkly colored, bitter, and slightly acidic, coffee has a stimulating effect on humans, primarily due to its caffeine content. It is the most popular hot drink in the world. Seeds of ...
, whereas grapefruit mercaptan, a monoterpenoid thiol, is responsible for the characteristic scent of
grapefruit The grapefruit (''Citrus'' × ''paradisi'') is a subtropical citrus tree known for its relatively large, sour to semi-sweet, somewhat bitter fruit. The interior flesh is segmented and varies in color from pale yellow to dark pink. Grapefruit ...
. The effect of the latter compound is present only at low concentrations. The pure mercaptan has an unpleasant odor. In the United States,
natural gas Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbon d ...
distributors were required to add thiols, originally ethanethiol, to
natural gas Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbon d ...
(which is naturally odorless) after the deadly
New London School explosion The New London School explosion occurred on March 18, 1937, when a natural gas leak caused an explosion and destroyed the London School in New London, Texas, United States. The disaster killed more than 300 students and teachers. , the event ...
in
New London, Texas New London is a city in Rusk County, Texas, United States. The population was 958 at the 2020 census. New London was originally known as just "London". Because Kimble County Texas had already established a US Post Office station named Londo ...
, in 1937. Many gas distributors were odorizing gas prior to this event. Most gas odorants utilized currently contain mixtures of mercaptans and sulfides, with ''t''-butyl mercaptan as the main odor constituent in natural gas and ethanethiol in liquefied petroleum gas (LPG, propane). In situations where thiols are used in commercial industry, such as liquid petroleum gas tankers and bulk handling systems, an oxidizing
catalyst Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
is used to destroy the odor. A copper-based oxidation catalyst neutralizes the volatile thiols and transforms them into inert products.


Boiling points and solubility

Thiols show little association by hydrogen bonding, both with water molecules and among themselves. Hence, they have lower
boiling point The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid varies depending upon the surrounding env ...
s and are less soluble in water and other
polar solvents A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
than alcohols of similar molecular weight. For this reason also, thiols and their corresponding sulfide functional group
isomers In chemistry, isomers are molecules or polyatomic ions with identical molecular formulae – that is, same number of atoms of each element – but distinct arrangements of atoms in space. Isomerism is existence or possibility of isomers. ...
have similar solubility characteristics and boiling points, whereas the same is not true of alcohols and their corresponding isomeric ethers.


Bonding

The S−H bond in thiols is weak compared to the O−H bond in alcohols. For CH3X−H, the bond enthalpies are for X = S and for X = O. Hydrogen-atom abstraction from a thiol gives a
thiyl radical In chemistry, a thiyl radical has the formula RS, sometimes written RS• to emphasize that they are free radicals. R is typically an alkyl or aryl substituent. Because S–H bonds are about 20% weaker than C–H bonds, thiyl radicals are relati ...
with the formula RS, where R = alkyl or aryl.


Characterization

Volatile thiols are easily and almost unerringly detected by their distinctive odor. Sulfur-specific analyzers for
gas chromatograph Gas chromatography (GC) is a common type of chromatography used in analytical chemistry for separating and analyzing compounds that can be vaporized without decomposition. Typical uses of GC include testing the purity of a particular substan ...
s are useful. Spectroscopic indicators are the D2O-exchangeable SH signal in the 1H NMR spectrum ( 33S is NMR-active but signals for divalent sulfur are very broad and of little utility). The ''ν''SH band appears near 2400 cm−1 in the IR spectrum. In the nitroprusside reaction, free thiol groups react with sodium nitroprusside and ammonium hydroxide to give a red colour.


Preparation

In industry, methanethiol is prepared by the reaction of
hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The under ...
with
methanol Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is ...
. This method is employed for the industrial synthesis of methanethiol: :CH3OH + H2S → CH3SH + H2O Such reactions are conducted in the presence of acidic catalysts. The other principal route to thiols involves the addition of hydrogen sulfide to
alkenes In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, a ...
. Such reactions are usually conducted in the presence of an acid catalyst or UV light.
Halide In chemistry, a halide (rarely halogenide) is a binary chemical compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative (or more electropositive) than the halogen, to make a flu ...
displacement, using the suitable organic halide and sodium hydrogen sulfide has also been utilized. Another method entails the alkylation of sodium hydrosulfide. : RX + NaSH → RSH + NaX(X = Cl, Br, I) This method is used for the production of thioglycolic acid from chloroacetic acid.


Laboratory methods

In general, on the typical laboratory scale, the direct reaction of a haloalkane with sodium hydrosulfide is ''in''efficient owing to the competing formation of sulfides. Instead, alkyl halides are converted to thiols via an ''S''-alkylation of thiourea. This multistep, one-pot process proceeds via the intermediacy of the
isothiouronium salt In organic chemistry, isothiouronium is a functional group with the formula SC(NH2)2sup>+ (R = alkyl, aryl) and is the acid salt of isothiourea. The H centres can also be replaced by alkyl and aryl. Structurally, these cations resemble guanid ...
, which is hydrolyzed in a separate step: : CH3CH2Br + SC(NH2)2H3CH2SC(NH2)2r : H3CH2SC(NH2)2r + NaOH → CH3CH2SH + OC(NH2)2 + NaBr The thiourea route works well with primary halides, especially activated ones. Secondary and tertiary thiols are less easily prepared. Secondary thiols can be prepared from the ketone via the corresponding dithioketals. A related two-step process involves alkylation of thiosulfate to give the thiosulfonate (" Bunte salt"), followed by hydrolysis. The method is illustrated by one synthesis of thioglycolic acid: :ClCH2CO2H + Na2S2O3 → Na 3S2CH2CO2H+ NaCl :Na 3S2CH2CO2H+ H2O → HSCH2CO2H + NaHSO4 Organolithium compounds and
Grignard reagent A Grignard reagent or Grignard compound is a chemical compound with the general formula , where X is a halogen and R is an organic group, normally an alkyl or aryl. Two typical examples are methylmagnesium chloride and phenylmagnesium bromide . ...
s react with sulfur to give the thiolates, which are readily hydrolyzed: :RLi + S → RSLi :RSLi + HCl → RSH + LiCl Phenols can be converted to the thiophenols via rearrangement of their ''O''-aryl dialkylthiocarbamates. Thiols are prepared by reductive dealkylation of sulfides, especially benzyl derivatives and thioacetals. Thiophenols are produced by ''S''-arylation or the replacement of diazonium leaving group with sulfhydryl anion (SH): : + SH → ArSH + N2


Reactions

Akin to the chemistry of alcohols, thiols form
sulfide Sulfide (British English also sulphide) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to chemical compounds la ...
s, thioacetals, and thioesters, which are analogous to ethers, acetals, and esters respectively. Thiols and alcohols are also very different in their reactivity, thiols being more easily oxidized than alcohols. Thiolates are more potent nucleophiles than the corresponding alkoxides.


''S''-Alkylation

Thiols, or more specific their conjugate bases, are readily alkylated to give sulfides: :RSH + R′Br + B → RSR′ + Br (B = base)


Acidity

Thiols are easily deprotonated. Relative to the alcohols, thiols are more acidic. The conjugate base of a thiol is called a thiolate. Butanethiol has a p''K''a of 10.5 vs 15 for butanol. Thiophenol has a p''K''a of 6, versus 10 for
phenol Phenol (also called carbolic acid) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile. The molecule consists of a phenyl group () bonded to a hydroxy group (). Mildly acidic, it r ...
. A highly acidic thiol is
pentafluorothiophenol Pentafluorothiophenol is an organosulfur compound with the formula C6F5SH. It is a colorless volatile liquid. The compound is prepared by the reaction of sodium hydrosulfide and hexafluorobenzene. With a pKa of 2.68, it is one of the most acidi ...
(C6F5SH) with a p''K''a of 2.68. Thus, thiolates can be obtained from thiols by treatment with alkali metal hydroxides.


Redox

Thiols, especially in the presence of base, are readily
oxidized Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...
by reagents such as
bromine Bromine is a chemical element with the symbol Br and atomic number 35. It is the third-lightest element in group 17 of the periodic table ( halogens) and is a volatile red-brown liquid at room temperature that evaporates readily to form a simi ...
and iodine to give an organic disulfide (R−S−S−R). : 2 R−SH + Br2 → R−S−S−R + 2 HBr Oxidation by more powerful reagents such as sodium hypochlorite or
hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3 ...
can also yield
sulfonic acid In organic chemistry, sulfonic acid (or sulphonic acid) refers to a member of the class of organosulfur compounds with the general formula , where R is an organic alkyl or aryl group and the group a sulfonyl hydroxide. As a substituent, it is k ...
s (RSO3H). : R−SH + 3 H2O2 → RSO3H + 3 H2O Oxidation can also be effected by oxygen in the presence of catalysts: : 2 R–SH +  O2 → RS−SR + H2O Thiols participate in thiol-disulfide exchange: :RS−SR + 2 R′SH → 2 RSH + R′S−SR′ This reaction is important in nature.


Metal ion complexation

With metal ions, thiolates behave as ligands to form
transition metal thiolate complex Transition metal thiolate complexes are metal complexes containing thiolate ligands. Thiolates are ligands that can be classified as soft Lewis bases. Therefore, thiolate ligands coordinate most strongly to metals that behave as soft Lewis acids as ...
es. The term ''mercaptan'' is derived from the
Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...
''mercurium captans'' (capturing mercury) because the thiolate group bonds so strongly with mercury compounds. According to hard/soft acid/base (HSAB) theory, sulfur is a relatively soft (polarizable) atom. This explains the tendency of thiols to bind to soft elements and ions such as mercury, lead, or cadmium. The stability of metal thiolates parallels that of the corresponding sulfide minerals.


Thioxanthates

Thiolates react with carbon disulfide to give
thioxanthate In chemistry, a thioxanthate is an organosulfur compound with the formula RSCS2X. When X is an alkali metal, the thioxanthate is a salt. When X is a transition metal, the thioxanthate is a ligand, and when X is an organic group, the compounds are ...
().


Thiyl radicals

Free radicals derived from mercaptans, called
thiyl radical In chemistry, a thiyl radical has the formula RS, sometimes written RS• to emphasize that they are free radicals. R is typically an alkyl or aryl substituent. Because S–H bonds are about 20% weaker than C–H bonds, thiyl radicals are relati ...
s, are commonly invoked to explain reactions in
organic chemistry Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, J ...
and
biochemistry Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology and ...
. They have the formula RS where R is an organic substituent such as
alkyl In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term ''alkyl'' is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of . A cycloalkyl is derived from a cycloa ...
or
aryl In organic chemistry, an aryl is any functional group or substituent derived from an aromatic ring, usually an aromatic hydrocarbon, such as phenyl and naphthyl. "Aryl" is used for the sake of abbreviation or generalization, and "Ar" is used ...
. They arise from or can be generated by a number of routes, but the principal method is H-atom abstraction from thiols. Another method involves homolysis of organic disulfides. In biology thiyl radicals are responsible for the formation of the deoxyribonucleic acids, building blocks for DNA. This conversion is catalysed by
ribonucleotide reductase Ribonucleotide reductase (RNR), also known as ribonucleoside diphosphate reductase (rNDP), is an enzyme that catalyzes the formation of deoxyribonucleotides from ribonucleotides. It catalyzes this formation by removing the 2'-hydroxyl group of t ...
(see figure). Thiyl intermediates also are produced by the oxidation of glutathione, an antioxidant in biology. Thiyl radicals (sulfur-centred) can transform to carbon-centred radicals via
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...
atom exchange equilibria. The formation of
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon ma ...
-centred radicals could lead to protein damage via the formation of C−C bonds or backbone fragmentation. Because of the weakness of the S−H bond, thiols can functioning as scavengers of free radicals.


Biological importance


Cysteine and cystine

As the functional group of the
amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha ...
cysteine, the thiol group plays a very important role in biology. When the thiol groups of two cysteine residues (as in monomers or constituent units) are brought near each other in the course of
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
folding, an oxidation reaction can generate a cystine unit with a disulfide bond (−S−S−). Disulfide bonds can contribute to a protein's tertiary structure if the cysteines are part of the same
peptide Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides. ...
chain, or contribute to the quaternary structure of multi-unit proteins by forming fairly strong covalent bonds between different peptide chains. A physical manifestation of cysteine-cystine equilibrium is provided by hair straightening technologies. Sulfhydryl groups in the active site of an
enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
can form noncovalent bonds with the enzyme's substrate as well, contributing to covalent
catalytic activity Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
in catalytic triads. Active site cysteine residues are the functional unit in
cysteine protease Cysteine proteases, also known as thiol proteases, are hydrolase enzymes that degrade proteins. These proteases share a common catalytic mechanism that involves a nucleophilic cysteine thiol in a catalytic triad or dyad. Discovered by Gopal ...
catalytic triads. Cysteine residues may also react with heavy metal ions (Zn2+, Cd2+, Pb2+, Hg2+, Ag+) because of the high affinity between the soft sulfide and the soft metal (see
hard and soft acids and bases HSAB concept is a jargon for "hard and soft (Lewis) acids and bases". HSAB is widely used in chemistry for explaining stability of compounds, reaction mechanisms and pathways. It assigns the terms 'hard' or 'soft', and 'acid' or 'base' to chem ...
). This can deform and inactivate the protein, and is one mechanism of heavy metal poisoning. Drugs containing thiol group
6-Mercaptopurine Mercaptopurine (6-MP), sold under the brand name Purinethol among others, is a medication used for cancer and autoimmune diseases. Specifically it is used to treat acute lymphocytic leukemia (ALL), acute promyelocytic leukemia (APL), Crohn's dis ...
(anticancer) Captopril (antihypertensive) D-penicillamine (antiarthritic) Sodium aurothiolate (antiarthritic)


Cofactors

Many
cofactors Cofactor may also refer to: * Cofactor (biochemistry), a substance that needs to be present in addition to an enzyme for a certain reaction to be catalysed * A domain parameter in elliptic curve cryptography, defined as the ratio between the order ...
(non-protein-based helper molecules) feature thiols. The biosynthesis and degradation of fatty acids and related long-chain hydrocarbons is conducted on a scaffold that anchors the growing chain through a thioester derived from the thiol Coenzyme A. The biosynthesis of
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane ...
, the principal
hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...
on Earth, arises from the reaction mediated by
coenzyme M Coenzyme M is a coenzyme required for methyl-transfer reactions in the metabolism of archaeal methanogens, and in the metabolism of other substrates in bacteria. It is also a necessary cofactor in the metabolic pathway of alkene-oxidizing bact ...
, 2-mercaptoethyl sulfonic acid. Thiolates, the conjugate bases derived from thiols, form strong complexes with many metal ions, especially those classified as soft. The stability of metal thiolates parallels that of the corresponding sulfide minerals.


In skunks

The defensive spray of skunks consists mainly of low-molecular-weight thiols and derivatives with a foul odor, which protects the skunk from predators. Owls are able to prey on skunks, as they lack a sense of smell.


Examples of thiols

* Methanethiol – CH3SH
ethyl mercaptan Ethanethiol, commonly known as ethyl mercaptan, is an organosulfur compound with the formula CH3CH2SH. is a colorless liquid with a distinct odor. Abbreviated EtSH, it consists of an ethyl group (Et), CH3CH2, attached to a thiol group, SH. Its st ...
* Ethanethiol – C2H5SH thyl mercaptan*
1-Propanethiol Propanethiol is an organic compound with the molecular formula C3H8S. It belongs to the group of thiols. It is a colorless liquid with a strong, offensive odor. It is moderately toxic and is less dense than water and slightly soluble in water. It ...
– C3H7SH 'n''-propyl mercaptan* 2-Propanethiol – CH3CH(SH)CH3
C3 mercaptan C3, C-3, C.3, C03, C.III or C-III may refer to: Life and biology * C3 carbon fixation, C3 carbon fixation in plants * C3-convertase, an enzyme * Complement component 3, a protein of the innate immune system * Apolipoprotein C3, a human very low d ...
* Allyl mercaptan CH2=CHCH2SH -propenethiol*
Butanethiol 1-Butanethiol, also known as butyl mercaptan, is a volatile, clear to yellowish liquid with a fetid (extremely foul-smelling) odor, commonly described as "skunk" odor. In fact, 1-butanethiol is structurally similar to several major constituents o ...
– C4H9SH 'n''-butyl mercaptan* ''tert''-Butyl mercaptan – (CH3)3CSH 't''-butyl mercaptan* Pentanethiols – C5H11SH entyl mercaptan* Thiophenol – C6H5SH *
Dimercaptosuccinic acid Succimer, sold under the brand name Chemet among others, is a medication used to treat lead, mercury, and arsenic poisoning. When radiolabeled with technetium-99m, it is used in a number of types of diagnostic testing. A full course is 19 day ...
* Thioacetic acid * Coenzyme A * Glutathione *
Metallothionein Metallothionein (MT) is a family of cysteine-rich, low molecular weight (MW ranging from 500 to 14000 Da) proteins. They are localized to the membrane of the Golgi apparatus. MTs have the capacity to bind both physiological (such as zinc, cop ...
* Cysteine * 2-Mercaptoethanol * Dithiothreitol/ dithioerythritol (an
epimer In stereochemistry, an epimer is one of a pair of diastereomers. The two epimers have opposite configuration at only one stereogenic center out of at least two. All other stereogenic centers in the molecules are the same in each. Epimerization is ...
ic pair) *
2-Mercaptoindole 2-Indolinethione is an organic compound with the formula . It is a derivative of dihydroindole containing a thione group. The compound is a tautomer of 2-mercaptoindole. A white solid, it is prepared by thiation of 2-oxindole. 2-Indolinethione ...
* Grapefruit mercaptan *
Furan-2-ylmethanethiol Furan-2-ylmethanethiol (2-Furanmethanethiol) is an organic compound containing a furan substituted with a sulfanylmethyl group. It is a clear colourless liquid when pure, but it becomes yellow coloured upon prolonged standing. It possesses a stro ...
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3-Mercaptopropane-1,2-diol 3-Mercaptopropane-1,2-diol, also known as thioglycerol, is a chemical compound and thiol that is used as a matrix in fast atom bombardment mass spectrometry and liquid secondary ion mass spectrometry. See also * Glycerol * Mercaptoethanol 2- ...
* 3-Mercapto-1-propanesulfonic acid * 1-Hexadecanethiol * Pentachlorobenzenethiol


See also

*
Doctor sweetening process The doctor sweetening process is an industrial chemical process for converting mercaptans in sour gasoline into disulfides. Sulfur compounds darken gasoline, give it an offensive odor and increase toxic sulfur dioxide engine emissions. However, th ...
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Odorizer An odorizer is a device that adds an odorant to a gas. The most common type is one that adds a mercaptan liquid into natural gas distribution systems so that leaks can be readily detected. Other types have been used for carbon dioxide fire e ...
* Persulfide * Saville reaction * Thiol-disulfide exchange


References


External links


Mercaptans (or Thiols)
at '' The Periodic Table of Videos'' (University of Nottingham)
Applications, Properties, and Synthesis of ω-Functionalized n-Alkanethiols and Disulfides – the Building Blocks of Self-Assembled Monolayers
by D. Witt, R. Klajn, P. Barski, B.A. Grzybowski at Northwestern University.

by '' The Columbia Electronic Encyclopedia''
What is Mercaptan?
by Columbia Gas of Pennsylvania and Maryland.

by About Chemistry. {{Authority control Functional groups Organosulfur compounds